Present systems for producing products such as sodium chlorate by the electrolysis of sodium chloride solutions may generally be classified in the following groups:
(a) Bipolar cells placed in large tanks; PA1 (b) Small monopolar cells.
Both of these systems have inherent disabilities. The bipolar cells which are situated in a large tank of electrolyte, are generally swept with air to dilute the evolved hydrogen below the explosive limit, generally 50 volumes of air for each volume of hydrogen. This combined air-hydrogen stream contains small particles of sodium chloride and sodium chlorate, and because of the large volume involved it is very difficult to effectively scrub out these particles. Because of the configuration of the cells it is difficult to direct the air stream to obtain adequate contact between the air and the solution and to achieve sufficient evaporation which would result in cooling and concentration of the solution. For this reason large quantities of cooling water are needed and large expensive heat transfer surfaces are involved. Due to insufficient concentration of the solution this must be sent through an evaporator for further concentration prior to use in any of the standard chlorine dioxide generators. This requires more energy usually in the form of steam.
The small monopolar cells also have their inherent difficulties. Each small cell must have either its individual acid and electrolyte feed and individual cooling or the electrolyte from many cells must be circulated to a large reaction tank and heat exchanger. This, of course, requires extra equipment and energy for pumping. In this procedure no air is swept across the electrolyte to cause evaporation, cooling and concentration, therefore this system also requires large quantities of cooling water and large heat transfer surfaces and in addition the solution requires further concentration before it is suitable for chlorine dioxide generator feed.
In pending U.S. application Ser. No. 574,391 filed May 5, l975 for an invention of Harold V. Casson, Richard E. Loftfield and Bruno Kindl, an electrolytic cell was described in which there was a cell chamber containing an electrode assembly of anodes and cathodes to electrolyze an electrolytic solution to generate a dissolved product and a gas. This gas is separately removed from the cell chamber. A cooling and concentration chamber, which, as known in this art, acts also as a reaction chamber where hypoxyhalides convert to halates, is also provided and this communicates with the cell chamber to allow for the free or forced circulation of electrolytic solution. However, there is a substantial seal against the passage of gas from one chamber to the other. A gas sweep is directed through the cooling and concentration chamber to cool the electrolyte and evaporate part of the electrolyte so as to concentrate it.
It has, however, been found that with the structure described in the application referred to above, there was a problem in providing access without undue difficulty to the cells, while making certain that there was a minimum risk of explosion as a consequence of the leakage of hydrogen from the cell chamber into the cooling and concentration chamber. The flammability limit of hydrogen in air is 4% by volume. There can also be an explosion at the opposite extreme of air in hydrogen mixtures if there is more than 25.8% of air in hydrogen (Handbook of Chemistry and Physics 54th Ed. 1973-74 pages D85-86, Chemical Rubber Company). It will therefore be apparent that there could be a danger of explosion in the event of a slight leakage.